1. P. A. Lee, Phys. Rev. B13 5261 (1978).
2. G. M. Lamble and D. A. King, Phil. Trans. Roy. Soc. Lond. A318 (1986) 203.
3. A. A. MacDowell, D. Norman, J.B. West, J.C. Campuzano and R.G. Jones, Nucl. Instrum.Meth. (in press): A.A. MacDowell, D. Norman and J.B. West, (to be published). Rev. Mod. Phys.
4. S. J. Gurman, N. Binstead, and I. Ross, J. Phys. C17 143 (1984).
5. P. A. Lee and J. B. Pendry, Phys. Rev. B 11, 2795 (1975).